Robust {Cd₄}-Organic Framework for Efficiently Catalyzing CO₂ Cycloaddition and Knoevenagel Condensation

Abstract

The high-value-added carbonates generated from CO2 have attracted the attention of more and more researchers because of which the optimization of metal–organic framework (MOF)-based catalysts has seen a considerable upsurge at present. The scarcely reported cadmium(II)-based MOFs inspire us to explore CdOFs with excellent catalytic activity and high reusability. Herein, the unification of the unreported {Cd4(μ3-OH)2(CH3CO2 –)} cluster and 2,6-bis(2,4-dicarboxylphenyl)-4-(4-carboxylphenyl)pyridine (H5BDCP) led to a highly robust nanoporous crystalline material of {(Me2NH2)5[Cd4(BDCP)2(μ3-OH)2(CH3CO2)(H2O)2]·3DMF·2H2O} n (NUC-67) with 57.4% void volume. Structural analysis displays that the inner surface of channels in activated NUC-67a is functionalized by Lewis acid sites of unsaturated Cd2+ ions and Lewis base sites of μ3-OH– groups, CH3CO2 – anions, free pyridine, and CO groups. Under solvent-free conditions, NUC-67a exhibits high catalytic performance on the cycloaddition of CO2 with epoxides; for instance, the conversion rate of propylene oxide (PO) into propylene carbonate (PC) with 1 atm CO2 can reach 99% within 6 h at 55 °C, resulting in a 660 turnover number and 110 h–1 turnover frequency. Moreover, Knoevenagel condensation reactions of aldehydes and malononitrile can be efficiently catalyzed by activated NUC-67a. Encouragingly, NUC-67a shows strong structural stability and good reversible cyclicity in the above two organic reactions with metal leaching below 8 ppb. Hence, this work proves that the optimization of MOF-based catalysts should focus on the design and selection of organic ligands, which plays a decisive role in structural regulation, such as cluster-based nodes, high defect of metal sites, unexpected insertion of Lewis base sites, and high-porosity channels.

Robust Nitro-Functionalized {Zn3}-Organic Framework for Excellent Catalytic Performance on Cycloaddition Reaction of CO2 with Epoxides and Knoevenagel Condensation

“…As summarized in Table and shown in Figures S10–S18, all reactions carried out under the optimum reaction conditions were completed in the mode of nearly complete conversion of the substrate molecules into corresponding cyclic carbonates, which should be due to that (i) the sufficiently large pore size in NUC-80a has very little resistance to the free mass transfer of all selected substrate molecules and products and (ii) the highly rich nitro group can help open Zn 2+ sites to polarize the epoxy alkane through the hydrogen bond force with the guest molecule of epoxides. , In order to detect the effect of substituents with different properties on such catalytic reaction, we shortened the reaction time to 3 h and checked the yield of corresponding cyclic carbonates. As given in entries 2–8, among all the selected epoxide derivatives, the reactants with electrophilic substituents have the highest yield (97%), while the ones with electron-donor groups display slightly reduced conversion (93–95%), consistent with relevant research reports that the electron absorption group can help the nucleophilic attack of bromine ion by reducing the electronic density of ethylene oxide. , As for oxidized styrene (OS), the resulting yield for product styrene carbonate (SC) is lowest. According to the latest relevant research, , this should be attributed to the steric hindrance (the size of epoxies in Table S3), which greatly hinders the rapid mass transfer of molecules in the channels of NUC-80a .…”

Robust Zn(II)-Organic Framework for High Catalytic Activity on Cycloaddition of CO2 with Epoxides and Reversible Iodine Uptake

“…Table S3 shows the molecular sizes of the epoxide derivatives involved. In addition, compared with recent literature on MOFs catalysts (Table S4), − NUC-69a exhibited better catalytic performance, which should be ascribed to the more accessible coexisting Lewis acid–base sites in its confined pore structures.…”

Thiazolo[5,4-d]thiazole-Based Metal–Organic Framework for Catalytic CO2 Cycloaddition and Photocatalytic Benzylamine Coupling Reactions

“…The possible catalytic mechanism was then proposed. − As shown in Figure , initially, O epoxide is activated by the active metal sites, which leads to the instability of the ternary ring, and then Br – of TBAB participates in the nucleophilic attack of the epoxy ring of the substrate, which opens the epoxy ring and forms a reactive intermediate. Subsequently, the active intermediate is combined with CO 2 activated by the LBSs (TZTZ) to generate alkyl carbonate anions, which again undergoes intramolecular ring closure to generate the final product cyclic carbonates and regenerate MOF-LS10 and the cocatalyst TBAB for the next step of the cycloaddition reaction.…”